In the electronics industry it is often desirable to cover or coat an existing refractory metal surface with a brazable or solderable surface. Applications for such a procedure, include but are not limited to, input/output (I/O) pads, wire bond pads, C4's (Controlled Collapse Chip Connection), seal bands, to name a few.
Many methods are available and practiced in the industry to cover or coat an existing refractory metal surface with a brazable or solderable surface. The most commonly used approach for treating refractory metal surfaces in the microelectronic packaging business is to employ electroplating or electroless-plating of pure or substantially pure Ni (nickel) film from an aqueous bath which is at or near room temperature.
Nickel is generally the metal of choice for plating refractory metals because it can be made to bond well with any of the refractory metals. In addition, Ni possesses good wetting characteristics for subsequent bonding processes, such as brazing or soldering, and it has excellent corrosion characteristics.
Recently, a few high temperature, dry, halide transfer processes have been disclosed and subsequently used by the industry for the purpose of plating nickel on refractory metal surfaces of electronic packages using molybdenum or tungsten.
One method disclosed in U.S. Pat. No. 4,590,095 (Park) uses a pack cementation approach. The essential elements for pack cementation are a powder metal source, an activator, and an object to be plated. Basically, the elements are placed in a chamber and the object is buried in a mixture of the powder metal source, activator, and usually an inert ceramic powder, such as, alumina, and then heated to a high temperature to establish vapor transport. The process allows for mass transfer of the gas species. For the Park process pure nickel powder was used as the metal source and the activator used was ammonium iodide.
A departure from this pack cementation approach for a halide transfer process was disclosed in U.S. Pat. No. 4,664,942 (Park). In this case ammonium iodide and pure nickel were still the essential elements for the halide transfer process. However, in this case nickel screens were used as the metal source rather than the nickel powder. And, the objects to be plated, containing exposed surfaces of refractory metal, were placed in stacks with the nickel screens acting as separators in the reaction vessel or work boat. The ammonium iodide activator for the process was held in a separate crucible within the work boat. The elements were again heated to a high temperature to establish vapor transport. The open nickel screen allowed for mass transfer of the gas species and also served as the nickel source.
Most recently, another improvement was put forward in U.S. Pat. No. 5,869,134 (Reddy et al.), filed issued Feb. 9, 1999, entitled "CVD OF METALS CAPABLE OF RECEIVING NICKEL OR ALLOYS THEREOF USING IODIDE", presently assigned to International Business Machines Corporation, and the disclosure of which is incorporated herein by reference, where CuI was disclosed as a preferred iodide activator providing various advantages.
Another improvement that has been proposed is in U.S. patent application Ser. No. 09/050,491, filed on Mar. 30, 1998, entitled "CVD OF METALS CAPABLE OF RECEIVING NICKEL OR ALLOYS THEREOF USING INERT CONTACT", presently assigned to International Business Machines Corporation, and the disclosure of which is incorporated herein by reference, where at least one inert material is in a floating contact with the receiving metal, and the inert material provides physical separation between the source metal and the receiving metal.
Dry Process (DP) nickel is basically a halide transfer process where nickel metal is transported in the gas phase from a solid nickel source and deposited as a solid metallic film on a refractory metal surface. The halide used in this case is iodide, i.e., iodide is the carrier gas. Ideally the reaction will be made to take place in a closed container which in general is not tightly sealed.
In general, for halide transfer metal deposition processes, it is very desirable that the metal source material, preferably, a substantially pure solid nickel source and the refractory metal area to be plated are kept in close physical proximity to each other. This close proximity condition is necessary in order to maintain a reasonable rate of metal deposition during the process.
In the prior art halide transfer processes, referenced above, the metal source material, powder or screens, were kept in close physical proximity to the refractory metal surface to be plated. However, in addition, due to the specific geometrical configuration of each assembly, the metal source material can at least at some point, also come into direct physical contact with the metal surface to be plated. It has been discovered that when the source metal and the target areas do touch each other, during the deposition process, while using these known processes, they, the source and the sink, can form a bond. When the nickel plated part and the other assembly materials are subsequently separated, after the deposition process has been completed, a defect in the deposited nickel film can be readily observed. This defect can take the form of a taffy pull of metal or a piece of metal debris or a missing section of the deposited nickel film, etc. This condition will normally, result in the rejection of the part or work piece.
Another improvement that has been proposed in U.S. patent application Ser. No. 09/050,490, filed on Mar. 30, 1998, entitled "CVD OF METALS CAPABLE OF RECEIVING NICKEL OR ALLOYS THEREOF USING INERT AN INERT STRUCTURE WITH EMBEDDED NICKEL OR ALLOYS THEREOF", presently assigned to International Business Machines Corporation, and the disclosure of which is incorporated herein by reference, where at least one inert structure with embedded nickel is in a floating contact with the receiving metal, and the inert material provides physical separation between the source metal and the receiving metal.
Thus there is a need for having at least one source metal that is embedded in at least one inert material to form a stand-alone structure, and to be able to use this structure in a CVD (Chemical Vapor Deposition) process where the source metal, such as, nickel or alloys thereof, such as, Ni/Cu, Ni/Co, are deposited on metal surfaces which are capable of receiving the source metal, such as, refractory metal, such as, molybdenum, tungsten or alloys thereof, using an Iodide source, preferably an Iodide salt, such as, Ammonium Iodide or Copper Iodide, where the source metal and a high strength inert material that provides separation between the source metal and the receiving metal surface are embodied in a single structure. This invention basically allows the CVD of nickel (Ni) or alloys thereof, on the surface of refractory metal, such as, molybdenum (Mo) or tungsten (W), where the nickel source is physically isolated from the refractory metal surface to be plated using at least one high strength inert material that is in physical floating contact with the refractory metal surface that needs to be coated with at least one layer of nickel or alloy thereof, and where the nickel source and the high strength inert material are embodied in a single bonded structure.
The present invention, further teaches a new material composition for the inert structure for improving the strength, porosity and hence the performance and life of these structures in the halide transfer process where nickel metal is electrolessly deposited onto a refractory metal surface. With the method of this invention the metal source and the refractory metal surface to be plated, are kept in close physical proximity, as required, to effect rapid deposition rate, but where at least one improved strength inert standoff material is in contact with the receiving metal surface and where the high strength inert standoff material embodies or is bonded or attached to the metal source in the process, such that the high strength inert standoff material and the metal source are handled conveniently as a single unit. The inert material continues to provide complete physical isolation between the source metal and the surface being plated, such that there does not exist any opportunity for the source metal and the receiving metal to touch and weld and form a bond creating a defect. However, in this present invention the source metal and the high strength inert material providing for isolation are securely bonded as a single unit, and may be referred to as high strength tile or high strength settertile.